A new study published Saturday in the British medical journal the Lancet found that tens of millions of people in Bangladesh have been exposed to poisonous levels of arsenic from contaminated groundwater. Bangladesh has struggled with arsenic in its water supply since a disastrous campaign in the 1970s to bring clean water to the county backfired horribly. Millions of tube wells were drilled to provide villagers with clean water, but many of them were dug into shallow layers of ground that had naturally occurring arsenic, contaminating the water. The Lancet study puts a figure on the toll from arsenic poisoning — and it’s astonishing. The study team, led by Dr. Habibul Ahsan of the University of Chicago, found that as many as 77 million people — half the population of crowded Bangladesh — may have been exposed to toxic levels of arsenic. Ashan and his colleagues followed nearly 12,000 Bangladeshis over the court of 10 years and found that more than 20% of deaths were caused by arsenic:

They found that in the top 25% of people with the highest arsenic exposure, the risk of dying during the six years increased by nearly 70% compared with people with low arsenic levels.

People who drank moderate levels of arsenic were more likely to die from chronic disease than those who took in an amount within World Health Organization recommendations of 10 micrograms per liter.

Chronic arsenic exposure can lead to heart disease and cancers of the liver, kidney, bladders and skin — but the researchers found that arsenic could hasten death even at relatively low levels of exposure:

Compared to those exposed to the lowest arsenic levels (less than 10 micrograms of arsenic per liter of water), those with levels of 10-50 micrograms had a 34% higher risk of death, and those with the highest level (between 150 and 864 micrograms) a 64% higher risk.

But even exposure at relatively lower levels carried a risk, a finding that is important for other countries — there are more than 70 of them around the world, including the United States, India and Mexico — that face a serious arsenic problem.

What’s going on in Bangladesh is a unique horror — the World Health Organization has called it the “largest mass poisoning of a population in history” — and it’s partially due to the fact that Bangladesh gets 90% of its water from the ground. But what’s going on there is hardly unique. More than 70 countries around the world have issues with arsenic in their groundwater — and it’s not just poor countries like Bangladesh. The U.S. has a problem as well, as this map from the Environmental Protection Agency shows, especially in parts of California and the Midwest:

The Lancet study is a reminder that environmentalism isn’t just about endangered species or deforestation or even climate change — healthy humans need a health planet as well.